The entire disclosure of Japanese Patent application No. 2006-247654, filed Sep. 13, 2006 is expressly incorporated by reference herein.
1. Technical Field
The present invention relates to a technology that controls the operations of various electro-optical elements, such as light-emitting devices made of organic EL (ElectroLuminescent) material.
2. Related Art
In electro--optical elements made of organic EL material, gray-scale level (typically, luminance) is changed in accordance with an electric current supplied thereto. A configuration for controlling the electric current (hereinafter, referred to as “driving current”) by means of a transistor (hereinafter, referred to as “driving transistor”) has been proposed. However, this configuration is disadvantageous in that electro-optical elements have nonuniform gray-scale levels due to individual differences in characteristics (particularly, in threshold voltage) among driving transistors. A configuration for compensating for differences in threshold voltage among driving transistors in order to suppress such nonuniform gray-scale levels is disclosed, for example, in U.S. Pat. No. 6,229,506 (FIG. 2) and Japanese Unexamined Patent Application Publication No. 2004-246204 (FIG. 5 and FIG. 6).
FIG. 16 is a circuit diagram showing a configuration of pixel circuit P0, which is disclosed in the above U.S. Pat. No. 6,229,506. As shown in FIG. 16, a transistor Tr1 is connected between the gate and drain of a driving transistor Tdr. In addition, one electrode L2 of a capacitive element C0 is connected to the gate of the driving transistor Tdr. A holding capacitor C1 is a capacitor that is connected between the gate and source of the driving transistor Tdr. On the other hand, a transistor Tr2 is a switching element, which is connected between a data line 14 and the other electrode L1 of the capacitive element C0 to switch between conduction and non-conduction therebetween. The data line 14 is supplied with an electric potential (hereinafter, referred to as “data electric potential”) VD in accordance with a luminance that is specified by an organic light-emitting diode element (hereinafter, referred to as “OLED element”) 110.
With the above configuration, firstly, the transistor Tr1 is changed to an on state by signal S2. When the driving transistor Tdr is diode-connected, the electric potential of the gate of the driving transistor Tdr converges on “VEL-Vth” (where Vth is a threshold voltage of the driving transistor Tdr). Secondly, the transistor Tr1 is brought into an off state, and the transistor Tr2 is then brought into an on state by signal S1 to conduct the electrode L1 of the capacitive element C0 with the data line 14. As a result of this process, the electric potential of the gate of the driving transistor Tdr varies by the level which is obtained by dividing the amount of change in the electric potential in the electrode L1 depending on a ratio of the capacitance of the capacitive element C0 to the capacitance of the holding capacitor C1 (that is, the level depending on the data electric potential VD). Thirdly, the transistor Tr2 is brought into an off state, and the transistor Tel is then brought into an on state by signal S3. As a result, a driving current that is not dependent on the threshold voltage Vth is supplied through the driving transistor Tdr and the transistor Tel to the OLED element 110. The configurations, disclosed in Japanese Unexamined Patent Application Publication No. 2004-133240 (FIG. 2 and FIG. 3, and the above Publication No. 2004-246204, basically have the same principle for compensating for the threshold voltage Vth of the driving transistor Tdr.
However, in any one of the above U.S. Pat. No. 6,229,506 and the publication No. 2004-246204, during a period when the OLED element 110 actually emits light (hereinafter referred to as “light emission period”), the transistor Tr2 is changed to an off state, causing the electrode L1 of the capacitive element C0 to enters an electrically floating state. Thus, during the light emission period, the voltage applied to the capacitive element C0 tends to fluctuate. For example, the electric potential of the electrode L1 may fluctuate because of a noise due to switching of the transistor Tr2. As the voltage applied to the capacitive element C0 thus fluctuates during the light emission period, the electric potential of the gate of the driving transistor Tdr and the driving current Iel corresponding to this electric potential fluctuate, thereby causing a variation in the luminance (nonuniform display, such as a crosstalk) of the OLED element 110.
When the capacitances of the capacitive element C0 and/or holding capacitor C1 are increased, it is tentatively possible to reduce the influence of the fluctuation in electric potential of the electrode L1 on the electric potential of the gate of the driving transistor Tdr. However, in this case, because there is a problem that the scale of the pixel circuit P0 increases due to the increase in capacitance, it cannot be a realistic solution under the present circumstances where high-resolution pixels are highly required.